Spectroscopic imaging combines digital imaging and molecular spectroscopy techniques, which can include Raman scattering, fluorescence, photoluminescence, ultraviolet, visible and infrared absorption spectroscopies. When applied to the chemical analysis of materials, spectroscopic imaging is commonly referred to as chemical imaging. Instruments for performing spectroscopic (i.e. chemical) imaging typically comprise an illumination source, image gathering optics, focal plane array imaging detectors and imaging spectrometers.
In general, the sample size determines the choice of image gathering optic. For example, a microscope is typically employed for the analysis of sub micron to millimeter spatial dimension samples. For larger objects, in the range of millimeter to meter dimensions, macro lens optics are appropriate. For samples located within relatively inaccessible environments, flexible fiberscope or rigid borescopes can be employed. For very large scale objects, such as planetary objects, telescopes are appropriate image gathering optics.
For detection of images formed by the various optical systems, two-dimensional, imaging focal plane array (FPA) detectors are typically employed. The choice of FPA detector is governed by the spectroscopic technique employed to characterize the sample of interest. For example, silicon (Si) charge-coupled device (CCD) detectors or CMOS detectors are typically employed with visible wavelength fluorescence and Raman spectroscopic imaging systems, while indium gallium arsenide (InGaAs) FPA detectors are typically employed with near-infrared spectroscopic imaging systems.
Spectroscopic imaging of a sample can be implemented by one of two methods. First, a point-source illumination can be provided on the sample to measure the spectra at each point of the illuminated area. Second, spectra can be collected over the an entire area encompassing the sample simultaneously using an electronically tunable optical imaging filter such as an acousto-optic tunable filter (AOTF) or a liquid crystal tunable filter (“LCTF”). Here, the organic material in such optical filters are actively aligned by applied voltages to produce the desired bandpass and transmission function. The spectra obtained for each pixel of such an image thereby forms a complex data set referred to as a hyperspectral image which contains the intensity values at numerous wavelengths or the wavelength dependence of each pixel element in this image.
The ability to improve discrimination testing of inks, stains, fibers and cloth as well as to improve visualization of fingerprints and thin layer chromatography plates are critical to the forensic analysis. Such testing often requires obtaining the spectrum of a sample at different wavelengths. Conventional spectroscopic devices operate over a limited ranges of wavelength due to the operation ranges of the detectors or tunable filters possible. This enables analysis in the Ultraviolet (UV), visible(VIS), near infrared (NIR), mid infrared (MIR) wavelengths and to some overlapping ranges. These correspond to wavelengths of about 180-380 nm (UV), 380-700 nm(VIS), 700-2500 nm (NIR) and 2500-25000 nm (MIR). Thus, to obtain a comprehensive analysis over a broad range of wavelengths (i.e., a hyperspectral image) more than one spectroscopic device must be applied. Such broad or extended ranges make application time-consuming and not often possible. The sample position and condition may be changed between the first analysis or a later analysis thereby lessening the ability to precisely correlate the spectra obtained at different wavelength ranges. There is a need for rapid, non-invasive instrument capable of operating at IR, NIR, visible, fluorescence, luminescence and Raman modes to provide hyperspectral imaging of the sample.